1. Field of the Invention
The present invention relates to a fixing device for image recording apparatuses, and more particularly to a fixing device for use in such an image recording apparatus like a printer, copying machine and facsimile apparatus capable of recording an image by thermally fixing toner on a sheet of paper.
2. Description of the Prior Art
A conventional image recording apparatus using toner fixes the toner image transferred onto a sheet of paper by using a fixing device including a heating roller and a pressure roller. In this process, the toner is fixed on the sheet by the heat from the heating roller while the sheet is under pressure between the two rollers.
However, when the sheet passes between the two rollers, as the temperature of the heating roller is higher than that of the pressure roller, the moisture of the face of the sheet in contact with the heating roller vaporizes sooner, and then the sheet tends to curl toward the heating roller. As a result, sheets are not neatly stacked on the sheet stacker, and accordingly become troublesome to handle.
A fixing device to solve this problem is disclosed in the Japanese Patent Laid Open No. 1-227178 published on Sep. 11, 1989. FIG. 1 shows a cross-sectional view of this fixing device. In FIG. 1, a sheet 100 onto which a toner image is transferred is fixed while being carried by a heating roller 61 and a pressure roller 62, and forwarded via a guide plate 70 to another heating roller 63 and a roller 64. The heating roller 63 is intended to correct the curl caused by the heat of the heating roller 61, and the curling force is cancelled by the two heating rollers 63 and 61 arranged opposite to each other with respect to the sheet. This fixing device, however, requires heat sources for both heating rollers 61 and 63, thereby consumes more electric power and accordingly is uneconomical. Therefore, this disadvantage had to be overcome.